A firetube steam boiler as illustrated in FIG. 1 is commonly known in the art as used for steam generation. A conventional firetube steam boiler 6 may have a Scotch Marine design. The boiler has a closed housing or tank in which water or another fluid is vaporized. The vaporized fluid exits the boiler at saturation temperature for use in various processes or heating applications. Water partially fills a boiler tank or housing with a small volume left above to accommodate steam.
Various heat sources for steam generation may be used, such as a product of combustion of any type of fossil fuel (in a gas, liquid or solid condition) or waste gases of any process. With the first case, different type of burners may be used to perform fossil fuel combustion in the furnace. In the last case, the device for steam production names as a Heat Recovery Steam Generator. Usually (for example, Scotch Marine design) the furnace is immersed in the same water-filled vessel where also the steam generation occurs. Hot flue gas passes are generated in the furnace and pass through tubes (also named as firetubes, because hot flue gas travels inside of the tubes) that extend through the same water-filled closed vessel as furnace.
Water in the vessel of a conventional boiler is always saturated and has an almost uniform temperature through the vessel volume. Usually fresh water enters into the vessel at temperature much less than saturation temperature. However, due to a small ratio of fresh water mass to the mass of water inside of vessel, the temperature uniformity has only local character and does not impact to the heat transfer intensity. The flue gas passes through the furnace and firetubes to an exhaust port, such that the heat transferred by convection and radiation from the flue gas to the saturated water generates steam. The steam then extracts from the top segment of the housing of the boiler for use as desired.
Firetube boilers may include several bundles of firetubes through which the flue gas travels back and forth in the housing. For example, if the boiler includes two bundles of firetubes, the flue gas passes in one direction through a first bundle of firetubes, and then in an opposite direction through the second bundle of firetubes. This is typically referred to as a “three-pass” boiler, since the furnace is used to organize fossil fuel combustion and is considered as a first pass before traveling through the firetubes.